Various types of liquid crystalline displays (display cells) require a homeotropic orientation of the liquid crystalline (LC) compound relative to the substrate plate carrying the electrode. In the case of a homeotropic orientation the molecules of the LC compound have a preferred direction which is perpendicular to the surface of the substrate plate. A homeotropic orientation is necessary, inter alia, for LC displays having dynamic scattering, ECB (electrically controlled birefringence) and cholesteric-nematic phase transitions. In European Patent Application No. 0173.369 which corresponds substantially to U.S. Pat. No. 4,674,842 which issued June 23, 1987 to the Applicants herein, it is described that an orientation layer of certain silane compounds brings about a homeotropic orientation.
An object in accordance with the invention is to provide a display cell the orientation layer of which has a multilayer structure which brings about a tilted orientation of the LC compounds.
For a proper understanding it is to be noted that an orientation layer does not have to be oriented itself. Such a layer imposes (induces) an orientation on molecules of a liquid crystalline compound.
In the case of a tilted orientation, the molecules of the LC compound are oriented at an orientation angle (tilt angle) to the surface of the orientation layer. The orientation angle is calculated relative to the surface of the orientation layer. A tilt angle of, in particular, 15.degree.-30.degree. is of substantial importance for the so-called STN displays (supertwist nematic) in which the liquid crystalline cell medium has a twisted configuration of 180.degree.-360.degree.. Twist is to be understood to mean herein the rotation of the direction (director) of the longitudinal axis of the molecules of the LC compound across the cell thickness. An interesting display having a twist of 270.degree. is known as supertwist birefringence effect cell (SBE). It is known from, for example, J. Appl. Phys. 56 (2), 15 July 1984, pp. 251-262, that in order to obtain a tilted orientation, an orientation layer of SiO is applied by means of vapor deposition at an angle of 80.degree.-86.degree. relative to the normal to the substrate surface. In other words, an oblique vapour deposition is effected. This process is very laborious and has the disadvantage that the process parameters are difficult to control. Since, the tilt angle depends upon the vapor-deposition conditions, a precise and uniform adjustment of the tilt angle has not heretofore been possible.
The display cell in accordance with the invention does not have this disadvantage. In particular, a further object of the invention is the provision of the possibility of precisely and uniformly adjusting any desired tilt angle.